1. Field of the Invention
The present invention relates to a humidity sensing device comprising a solid electrolyte. More particularly, the present invention relates to an impedance type humidity sensing device comprising a solid electrolyte evidencing proton conductivity.
2. Description of the Prior Art
Heretofore, the use of solid electrolyte humidity sensors as a means for monitoring or controlling the environment has been limited. These devices typically provide an electrical signal which may be potentiometric, amperometric or conductometric in nature in response to the level of humidity in the atmosphere. Among the devices proposed for this purpose are the galvanic cell type humidity sensors which either employ proton or oxide ion conducting electrolytes as humidity sensing elements. Additionally, impedance type humidity sensors may be employed for this purpose. The electromotive force evidenced by such cells typically follows Nernstian behavior which serves as a calibration curve for the sensor. The proton or oxide ion conducting solid electrolyte chosen for use in such devices then becomes the prime factor in the construction of such humidity sensors. Workers in the art selected sintered perovskite-related phases in the barium or strontium cesium yttrium oxide family (MCe.sub.1-x Y.sub.x O.sub.3 M.dbd.Ba or Sr!) for this purpose. However, studies have revealed that electronic and/or proton ion conduction in these materials results in significant deviations from Nerstian behavior, so imposing additional calibration requirements. Accordingly, workers in the art have focused their interest upon alternative materials in their quest to find humidity sensing properties which will satisfy their needs.
Numerous references disclose gas sensors and humidity sensitive devices. However, none of these references disclose or suggest the specific galvanic type sensor described herein. Typical of the prior art references are the following:
Nakamura, et al., U.S. Pat. No. 4,024,036, discloses a proton permselective solid-state member formed of a heteropoly acid represented by the generic formula, H.sub.m X.sub.x Y.sub.y O.sub.z !nH.sub.2 O or a salt thereof In this formula, X represents at least one member selected from the group consisting of boron, aluminum, gallium, silicon, germanium, tin, phosphorous, arsenic, antimony, bismuth, selenium, tellurium, iodine and the first, second and third transition metals, Y represents at least one member selected from the first, second and third transition metals, provided that X and Y do not represent the same substance, m, x, y, z and n each represents a positive numerical value. The permselective member can be used as an electrolyte in a fuel cell and as a membrane in a hydrogen gas refining system.
Murata, et al., U.S. Pat. No. 4,497,701, discloses a humidity sensitive device comprising an insulated substrate, first and second electrodes formed on the surface of the insulating substrate and spaced apart from each other, and a humidity sensitive film formed on the surface of the insulating substrate and covering the surface of the substrate between the electrodes. It includes a conductive powder or a semi-conductive powder, a solid electrolyte powder and an organic polymer, at least part of which is cross-linked by a zirconium compound, which serves as a cross-linking agent to form a bridge to the organic polymer and to make the structure of the humidity sensitive film stable. Additionally, the zirconium compound increases the variation rate of the resistance value as a function of moisture absorption. Thus, the range of the resistance value can be made large and the humidity sensitive device can be used as a dew sensor.
Roy, et al., U.S. Pat. No. 4,587,172, discloses a low expansion ceramic material having the molecular formula I(Na)j(Zr.sub.2 -.sub.z Na.sub.4z) k(P.sub.3 -.sub.x Na.sub.x Si.sub.x) O.sub.12. This composition evidences a low thermal expansion and may be used in low expansion optical reflective structures. Such structures have an optically reflecting film deposited on a ceramic substrate having a very small thermal coefficient of expansion.
Yamai, U.S. Pat. No. 4,751,206, discloses a method of making a low thermal-expansive zirconyl phosphate ceramic, (ZrO).sub.2 -P.sub.2 O.sub.7. The method involves sintering a fine-powder compact of zinc oxide, magnesium oxide, bismuth oxide, manganese oxide, iron oxide, cobalt oxide, or nickel oxide, at a temperature ranging from 1200.degree. C. to 1700.degree. C. The resulting ceramic has a low thermal expansion coefficient.
Yamazoe, et al., U.S. Pat. No. 4,718,991, relates to proton gas sensors and a method for the use thereof in detecting gasses in oxygen containing ambients. The described sensor comprises three electrodes, an ionization electrode, a reference electrode and a detection electrode, each of which is connected to a proton conductor. Upon short circuiting of the ionization and reference electrodes, a measurement of the difference of potential across the detection electrode is made, thereby indicating the presence of gas.
Yamai, et al., U.S. Pat. No. 4,751,206, discloses a low thermal expansion material, potassium zirconium phosphate. This material has high strength and high thermal shock resistance. This product may be used for furnace refractories which are subject to thermal shock and as thermal shielding materials such as protective tiles on space vehicles which shield the vehicle from the heat of re-entry to the atmosphere.
Kawae, et al., U.S. Pat. No. 4,961,957, discloses an electrochemical cell having a solid electrolyte body and a plurality of electrodes formed thereon. At least one of the electrodes is porous, for use in determining the concentration of a subject gas in an atmosphere. The porous electrode may be comprised of platinum, an alloy of platinum, or another metal such as nickel, silver, gold, rhodium, palladium, iridium or ruthenium. The solid electrolyte body used as an oxygen sensor is formed of an oxygen-ion conductive solid electrolyte which includes ZrO.sub.2 (zirconia) as a major component, and at least one additive such as Y.sub.2 O.sub.3, CaO, Yb.sub.2 O.sub.3, and MgO.
Ammende et al., U.S. Pat. No. 4,976,991, discloses a hydrogen sensor having a solid electrolyte comprised of nasion, titsicon, khibinskite, wadeite or .beta.-Al.sub.2 O.sub.3. The electrodes are formed of platinum, palladium or palladium oxide.
U.S. Pat. No. 3,276,910 to Grasselli, et al., discloses an ion transfer medium for an electrochemical reaction apparatus for converting chemical energy into electrical energy. The invention employs a solid ion transfer medium. The device includes a non-conductive housing, with a solid ion-exchange membrane of a polymeric salt of a Group IV metal in an acid selected from the group consisting of phosphoric and arsenic acids positioned within the housing, electrodes positioned on the solid ion exchange membrane, means for introducing a gaseous fuel into contact with one side of the membrane, and means for introducing an oxidant onto the other side of the membrane and electrical conductors extending from the electrodes to the exterior of the housing.
U.S. Pat. No. 5,133,857 to Alberti et al., discloses a solid-state sensor for determining the concentration of gases that can react with hydrogen. The device includes a solid-state proton conductor having a reference electrode on one side thereof and an electrode which catalyses the reaction of the gas to be detected. The sensor is connected to a power feed system which supplies a current or voltage impulses. Also included is a system which detects the value of the potential after each impulse. This device can be operated at room temperature.
Alberti, et al., discloses an oxygen conductor, not a proton conductor. The specific composition of this oxygen conductor is described as zirconium hydrogen phosphate or zirconium triphosphate doped with silicates, such as H.sub.3 Zr.sub.2 PO.sub.4 (--SiO.sub.4).sub.2 (column 2, lines 57-60).